Cryogenic fluids such as liquid hydrogen, liquid oxygen, liquid natural gas and others are used in applications such as the propulsion of vehicles, including space craft operating in zero gravity conditions. Accurate measurement of the quantity of cryogenic fluid within a storage vessel is made difficult by the fact that such fluids are not stationary within the vessel, and can contact one or more areas of the inner surface of the vessel or remain completely out of contact with the vessel wall.
One cryogenic fluid sensor is disclosed, for example, in U.S. Pat. No. 5,393,736 which describes a sensor employing a high temperature ceramic superconducting material capable of measuring the change in electrical resistance as it is submerged in a liquified cryogen. Other cryogenic liquid level sensors operate using pressure sensors, thermocouple sensors, or other point sensors such as wound resistor elements in linear carbon resistors. All of these types of sensors indicate a change in temperature or pressure when the liquid level changes, and covers or uncovers the sensor.
One problem with sensors of the type described above is that they cannot operate in zero gravity conditions because the fluid level is sensed in accordance with its relative position to a fixed position sensor. As noted above, cryogenic fluid can "float" or freely move about the interior of a vessel, particularly in zero gravity conditions. Consequently, the fluid level cannot be accurately determined with respect to a fixed point of reference. Additionally, such prior sensors do not have the capability of sensing the quantities of different fluids which may be present in the same vessel, i.e., a liquid and a gas, two different liquids or two different gases.